Table class Table 是一个 sst 文件在内存中的表示，用户通过使用 Table 提供的 Iterator 来读取 sst 文件中存储的 kv 数据。Table 通过不断的读取并解析 sst 文件，来实现 Iterator 所提供的接

read or write files SequentialFile（线程不安全，顺序读）// A file abstraction for reading sequentially through a file class LEVELDB_EXPORT SequentialFile { public: SequentialFile() = default; SequentialFile(const SequentialFile&) = delete; SequentialFile& operator=(const SequentialFile&) = delete; virtual ~SequentialFile(); // Read up to "n" bytes from the file. "scratch[0..n-1]" may